A scanner system is a device in which a rotating shaft supporting a galvanometer mirror (hereinafter referred to as “mirror”) is rotated by a motor so that the reflective surface of the mirror is positioned at a desired angle. Thus, for example, a piece to be machined is irradiated at a predetermined position with a laser beam output from a laser oscillator. The scanner system is used in a laser perforating machine (hereinafter referred to as “laser machining apparatus”) for manufacturing printed circuit boards.
In the laser machining apparatus, the speed (responsiveness) to position the mirror and the error thereof with respect to a commanded value have enormous influence on the machining throughput and the machining position accuracy. The machining throughput of the laser machining apparatus is typically not lower than 30,000 holes per minute (not lower than 500 holes per second), and the mirror is positioned in an average time of 1 ms or shorter. On the other hand, the machining position error is not larger than ±15 μm in the laser machining apparatus as a whole. In the whole error, the allocatable error to the scanner system is about several micrometers.
In order to achieve such a high speed and such a high accuracy in positioning the laser beam, the scanner system has a servo control unit for feedback control of the angle of the mirror.
An angle sensor for detecting the rotation angle of the rotating shaft is attached to the rotating shaft supporting the mirror. In the mirror positioning operation, the servo control unit operates so that a tracking error with respect to the commanded value is zero. The commanded value is a fixed value at a target angle position of the mirror. Therefore, in order to make the steady-state error zero, a so-called type-1 servo system is constituted by using a servo compensator having an integral characteristic in a low frequency range. The servo control system may be implemented by an analog servo system to be controlled by continuous time control with an analog operational circuit, or a digital servo system to be controlled by discrete time control with a program of a microprocessor. Further, if necessary, the both may be used together.
In the laser machining apparatus, the irradiation positions of a laser beam are converted into the target angle positions of the mirror based on the coordinate data of holes to be machined. An upper control unit performs this coordinate conversion, and sends the commanded values to the servo control unit. In addition, in order to synchronize the irradiation of the laser pulse with the positioning of the mirror, the upper control unit controls the timing when the commanded values are sent and the timing when the laser oscillator is actuated.
In the background art, there is disclosed a technique for controlling a scanner system in which the settling time is adjusted to suppress such a resonance that the operating frequency of the scanner system enters the resonance frequency band of the scanner system (JP-A-2000-28955).
In addition, there is disclosed a technique in which a servo control unit is constituted by a compensator of an analog circuit and a compensator of a digital computer, and the compensator of the analog circuit acts as a notch filter at the torsional natural frequency of a scanner so as to expand the control bandwidth (JP-A-2002-196274).
The servo control unit has to move and settle the mirror in a predetermined positioning time in response to a series of the commanded values (hereinafter referred to as “angle command pattern”) sent in turn from the upper control unit. That is, in order to attain high-speed and high-accuracy laser machining, it is essential not only to make the steady-state error of the mirror angle zero simply but also to make the transient error of the settling operation (hereinafter referred to as “settling response”) as small as possible, while a laser pulse is shot as soon as the error enters predetermined tolerance.
The servo control system has a plurality of natural modes defining its dynamic characteristic. Each natural mode of a dynamic system is characterized by the natural frequency of vibration and the damping ratio of the vibration, while each a periodic damping mode is characterized by its time constant.
In the case of the servo control system of the scanner system, the natural modes of the system as a whole is determined by the structural vibration characteristics of the scanner system to be controlled or the dynamic characteristics of the servo compensator. A mode having a low natural frequency or a long time constant has an influence on the low frequency characteristic of the frequency response transfer function of the servo control system. On the other hand, a mode having a high natural frequency or a short time constant has an influence on the high frequency characteristic. Particularly in the high frequency range, there are some modes caused by the structural vibrations, whose resonance points may be close to each other.
Further, the angle command pattern is not uniform, but is varied in mirror travel angle or time interval. The excited modes are different depending on the angle command patterns. Thus, the settling response varies. Accordingly, in order to improve the responsiveness of the scanner system, it is necessary to design the scanner system not to be affected by the variety of the angle command pattern.